1. Field of the Invention
The present invention relates to a method for controlling a shape measuring apparatus.
2. Description of Related Art
There has been known a shape measuring apparatus which measures a shape of an object to be measured by moving a stylus head while scanning a surface of the object to be measured (for example, JP 2008-241420 A, JP 2013-238573 A, and JP 2014-21004 A).
To perform scanning measurement, a path for the scanning measurement needs to be generated.
The device disclosed in JP 2008-241420 A converts a design data (for example, non-uniform rational B-spline (NURBS) data) based on CAD data into a group of polynomials curves in a predetermined degree.
CAD data (for example, NURBS data) including path information is received from an external CAD system or the like, and the CAD data is converted into data of a group of points. The data at each point is combined data of coordinates (x, y, z) and normal line directions (P, Q, R). (That is, the data indicates (x, y, z, P, Q, R).)
In this description, the data having information of (x, y, z, P, Q, R) is referred to as contour point data.
Next, the coordinates at each point are offset by a predetermined amount in the normal line direction. (The predetermined amount is, particularly, a stylus head radius r—an amount of deflection Ep.) The data of a group of points calculated in this manner is referred to as “offset contour point data”.
Then, the offset contour point data is converted into a group of polynomials curves in a predetermined degree.
Here, it is assumed that the polynomials is a cubic function, and the curves are parametric cubic curves (PCC).
Based on the PCC curve, a path to measure a workpiece is generated.
Furthermore, a PCC curve is divided into a group of divided PCC curves. A speed curve is calculated from the group of divided PCC curves, and then, a moving speed (moving vector) of a probe is calculated. (For example, a moving speed (moving vector) of a probe is set based on a curvature of each segment of the group of divided PCC curves or the like.)
The probe is moved according to the moving speed calculated in the above manner, and a stylus head is moved while scanning a surface of an object to be measured (passive nominal scanning measurement: note that the word “nominal” in this description means scanning along a predetermined trajectory calculated in advance based on design data of an object.).
Furthermore, there has been known a method to perform scanning measurement while correcting a trajectory by continuously calculating a deflection correcting vector so that an amount of deflection of a probe becomes constant (JP 2013-238573 A).
In this description, such scanning is referred to as “active nominal scanning measurement”.
Moreover, there has been known a method to perform scanning measurement while generating a trajectory without using design data (autonomous scanning measurement, for example JP 5089428 B).
As described above, there are three measurement methods of passive nominal scanning measurement, active nominal scanning measurement, and autonomous scanning measurement, and each of them has merits and demerits.
For example, although all workpieces could be measured by the autonomous scanning measurement, the autonomous scanning measurement takes a long time.
For example, a moving speed of a probe in the autonomous scanning measurement is about 10 mm/sec to 15 mm/sec, and a moving speed of a probe in the nominal scanning measurement is about 50 mm/sec to 100 mm/sec. Thus, it is expected that the autonomous scanning measurement takes time about ten times longer than the nominal scanning measurement.